The present invention relates to a safety mechanism for use on a vertical reciprocating conveyor that moves cargo between two or more different vertical levels. More specifically, the present invention relates to a safety mechanism that is operable to lock the carriage of the vertical reciprocating conveyor in a fixed vertical position when the vertical speed of the carriage exceeds a preselected upper speed limit due to an uncontrolled descent of the carriage.
A typical vertical reciprocating conveyor includes a carriage having a platform to support cargo as the carriage is guided for vertical movement on a supporting structure or frame that includes at least a pair of vertical support columns. A typical mechanically operated vertical conveyor includes an electric motor that is connected through a lift chain to the carriage. Operation of the reversible electric motor acts on the lift chain to raise and lower the carriage along the spaced vertical support columns. When the carriage reaches a desired vertical level, a braking mechanism included in the electric drive motor locks the motor in place and prevents further rotation of the lift chain and thus prevent descent of the carriage due to gravity.
It has been recognized that unintentional descent of the carriage from an elevated level can create a severe safety hazard. Unintentional descent of the carriage can be the result of overloading the carriage, either intentionally or inadvertently, beyond its rated capacity. One type of possible failure in the lifting mechanism for the carriage is the complete failure of the lift chain used to lift the carriage between the upper and lower levels.
In the past, attempts have been made to include a safety mechanism with the vertical conveyor to provide protection against uncontrolled descent of the carriage. For example, one type of prior art safety mechanism includes a safety cam that is held in a non-engaging release position by the weight of the carriage acting on the lift chain. If the lift chain fails, the release of the weight of the carriage on the lift chain allows a torsion spring to urge the safety cam into a locking position and engage one of the vertical uprights forming the frame of the vertical conveyor. The mechanical interaction between the safety cam and one of the vertical supports locks the carriage in a stationary position to prevent further uncontrolled descent of the carriage.
Although the safety mechanism described above functions well if the uncontrolled descent of the carriage is due to a failure in the lift chain, other types of failure in the conveyor can also cause uncontrolled descent of the carriage. For example, the most likely type of failure in the conveyor that would cause an uncontrolled descent is a failure in the braking mechanism of the drive motor. If the braking mechanism fails, the carriage can fall from an elevated position although no damage has been done to the lift chain. If the braking mechanism fails, tension will remain in the lift chain and the safety mechanism described above will not activate to prevent uncontrolled descent of the carriage.
Therefore, it is an object of the present invention to provide a vertical-reciprocating conveyor that includes a safety mechanism that prevents uncontrolled vertical movement of the carriage. Further, it is an object of the present invention to provide a safety mechanism that prevents uncontrolled movement of the carriage when the speed of the carriage exceeds a predetermined, upper speed limit. It is an additional object of the present invention to provide a safety mechanism that includes a locking device that is released from a non-engaging position to an engaging position when the speed of the carriage exceeds the upper speed limit. Additionally, it is an object of the invention to provide a safety mechanism that includes a speed-sensing device that acts as an escapement that has a fail-safe mode of operation.
The present invention is directed to a safety mechanism for automatically locking a vertical reciprocating conveyor at an elevated level upon detection of an uncontrolled descent of the carriage. The vertical reciprocating conveyor includes a carriage having a loading deck adapted to support a load or cargo. The carriage is guided for vertical movement between at least a pair of vertical support columns. The carriage is moved between at least an upper and a lower level by a drive mechanism that includes a reversible drive motor connected to the carriage by at least a pair of lift chains.
The safety mechanism of the present invention includes a locking device mounted to each side frame member of the carriage. Each locking device includes a cam that is rotatably movable between a non-engaging position and an engaging position. The cam is biased into the engaging position by a torsion spring. When the cam is in the engaging position, a series of serrated teeth on the cam engage an inside wall of one of the vertical support columns to lock the carriage in a vertical position.
The cam of the locking device is normally held in its non-engaging position by an engagement pin. The engagement pin contacts a portion of the cam to hold the cam in the non-engaging position against the bias force generated by the torsion spring. When the cam is in the non-engaging position, the carriage is freely movable along the spaced vertical support columns.
The safety mechanism further includes a speed-sensing device positioned to detect the speed of vertical movement of the carriage and is operable to release the locking device from the non-engaging position to the engaging position when the speed of the carriage exceeds a predetermined upper speed limit. The speed-sensing device includes a hollow, tubular housing and a sprocket mounted to a common shaft that is rotated by a safety chain that passes around the sprocket and is connected to the moving carriage. As the carriage moves vertically along the vertical support columns, the safety chain rotates the sprocket, the shaft and the attached tubular housing at a speed directly proportional to the vertical speed of the carriage.
A weighted shaft is positioned within the tubular housing and includes a pair of end caps connected to opposite ends of the weighted shaft. The end caps attached to each end of the weighted shaft are larger than the diameter of the tubular housing such that the weighted shaft is entrapped within the tubular housing. The movement of the weighted shaft into and out of the hollow housing is limited by each of the end caps.
A abutment is spaced above the rotating tubular housing by a distance sufficient to allow the tubular housing and weighted shaft to rotate beneath the abutment during the normal operating condition of the vertical reciprocating conveyor. As the speed-sensing device rotates, the force of gravity causes the weighted shaft to move into and out of the hollow housing. During the normal operating conditions, the and cap attached to the weighted shaft passes beneath the abutment which allows the housing and sprocket to freely rotate.
When the speed of the carriage increases, the weighted shaft remains extended from the tubular housing as the extended portion of the weighted shaft rotates past a horizontal equator extending through the rotational axis for the housing. The speed-sensing device is designed such that when the speed of rotation of the sprocket and tubular housing exceeds a preselected upper speed limit, the weighted shaft contacts the abutment and prevents further rotation of the sprocket.
The speed sensing device acts as an escapement that has a fail-safe mode of operation. Since the weighted shaft must move into the hollow housing before passing beneath the abutment, the escapement must be operating properly in order for the carriage to continue moving along the vertical support columns. If the escapement fails, the weighted shaft will remain extended from one end of the tubular housing and thus contact the abutment during normal operation of the vertical reciprocating conveyor. Thus, if the escapement fails, the weighted shaft will contact the abutment and prevent further rotation of the sprocket.
When the rotation of the sprocket is stopped, the safety chain can no longer move with the falling carriage. When the safety chain is prevented from moving, the safety chain applies an upward force to the engagement pin holding the cam of the locking device in its non-engaging position. When the engagement pin is pulled upward by the safety chain, the cam is released and the bias force created by the torsion spring moves the cam into its engaging position. When in the engaging position, the serrated teeth formed on the cam engage an inside wall of one of these vertical support columns and locks the carriage in a stationary, vertical position.
The safety mechanism of the invention triggers operation of the locking device by detecting the speed of movement of the carriage. The uncontrolled descent of the carriage along the vertical support columns can be caused by several types of failures within the vertical reciprocating conveyor. For example, failure of the lift chains or failure of the braking mechanism in the drive motor can cause uncontrolled descent of the carriage, which then triggers the locking device. Therefore, the present invention activates the safety mechanism not only upon breakage of the lift chain but also upon failure of the braking mechanism in the drive motor.
Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings.